Phase-contrast imaging is an imaging method commonly used for high resolution and biological imaging in TEMs. Phase-contrast imaging produces two-dimensional phase-resolved images based on the phase shifts/delays experienced by electron wave fronts as they pass through various parts of a material, i.e. the sample/specimen). Because a perfect, focused lens does not produce phase contrast, the technique typically requires that images be taken slightly out of focus, or with an imperfect lens, or (more rarely) by using holographic techniques. These methods, however, are known to have significant drawbacks in terms of the phase contrast transfer function (“CTF”), such as the loss of information at low spatial frequencies, and contrast reversals and oversensitivity to incoherence at high spatial frequencies.
Other prior art methods of phase-contrast imaging have included the use of phase plates, which function to delay/shift the electron beam (in the axial direction thereof) by producing a 90 degree phase shift between low-angle and high-angle scattered electrons, and which are typically provided as a physical structure positioned at the back focal plane of the objective lens. Mathematically, the back focal plane corresponds to a two-dimensional Fourier space characterizing the diffraction or the spatial frequency. Thus, the manipulation of frequency components at the back focal plane by the phase plates is equivalent to spatial filtering that is in turn able to manage phase contrast. For example, one known type of phase plate is a Boersche phase plate, which comprises a pair of ring electrodes separated by a dielectric with an aperture in the center of the inner electrode for electrons to pass through. In this case, low-angle scattered electrons passing through the aperture in the center of the inner electrode experience a 90 degree phase shift, while high-angle scattered electrons are not considered. Another similar phase plate is known as a Ziernike phase plate having a thin film with a central aperture. Similar to the Boersch phase plate, the low-frequency limit is determined by the size of the aperture in the center.
Many of the prior art methods using a physical phase plate have significant drawbacks in terms of the CTF, such as the loss of information at low spatial frequencies and contrast reversals and oversensitivity to incoherence at high spatial frequencies. Physical phase plates, which produce a 90 degree phase shift between low-angle and high-angle scattered electrons, can bypass some of these problems but introduce problems of their own. For example, the physical phase plate itself can cause incoherent scattering, and intermediate spatial frequency information may be totally lost. Moreover, the physical phase plate may experience contamination. Despite these limitations, however, phase plates are considered as an important advance for improving the CTF curves in high-resolution and biological TEM. However, there is still a need for a phase plate system that does not suffer from these limitations.